Radar systems used for surveillance and tracking generally require some type of target processing for background estimation, detection threshold and constant false alarm control. In air target radars, this has typically been achieved with range-only constant false alarm rate (“CFAR”) processing or range and velocity CFAR processing or “area CFAR.”
Contemporary radars frequently accomplish the CFAR function with clutter maps containing running averages of received power for every multidimensional resolution cell, i.e. every range-azimuth-Doppler cell. The clutter map is then used to establish detection thresholds whereby new data is compared to the running average in each cell. In effect the compare process identifies target returns that have magnitudes above specified values when compared to a running average. A map-based CFAR threshold requires that a target have sufficient velocity to move it from resolution cell to resolution cell, from scan to scan. Although an air target is present only briefly within a given cell, it significantly contributes to the averaging process of the cell in which it is located. Generally, establishing cells sufficiently small allows air targets to have velocity sufficiently high to make the movement of the target progress from cell to cell from scan to scan. However, defining cells sufficiently small, to allow air targets to have velocity sufficiently high, fails for detection of hovering craft, such as helicopters, and most sea targets where the target velocity is relatively low or virtually zero. In this case, a clutter map actually averages, absorbs and reflects the target return, such that the target becomes its own background. This is known as “mapping out.” Efforts to detect a hovering craft, using the relatively high Doppler responses from the rotating blades of a helicopter for example, simply do not generally succeed, because the blades map out with the fuselage. Based on the foregoing, a fundamental problem exists when using a map based CFAR detection for slow or non moving targets.